I only have two dogs, so I walk them both together most of the time.
When I have a foster dog, I will usually walk the foster separately so we can work on training throughout the walk. Sometimes I'll walk the foster together with one of my dogs if I want my dog to model something for the foster's benefit (such as "it's okay to potty outside, really, I promise").
Walking three dogs at the same time is a pain in my butt and I rarely do it. I can, it's just kind of annoying.
ARCHMX TDCH Pongu the Insane, CD-C, RE, RL1X6, RL2X5, RL3X2 (GSD mix, b. Apr 2010)
Crookytail the Tigerwuff, RL1, ITD (Akita mix, b. Jan 2011)